Please welcome back guest blogger Sue Carney. Sue is my DNA expert.
In my previous posts and in Cold Case Live Chats, a few other DNA techniques have been mentioned. It seems appropriate to provide a little detail on these techniques: How they work and when best to use them.
Cells contain a number of well defined internal structures, collectively termed organelles. The nucleus of a cell is one example of an organelle. It is here that the DNA most commonly profiled for forensic purposes, can be found. Mitochondria, another type of organelle, exist in abundance within the cell – there may be more than a hundred, acting as powerhouses, converting sugar into forms of energy that the cell can use. Mitochondria contain their own DNA which can also be forensically profiled. The mitochondrial DNA (mtDNA) exists in circular form, similar to the arrangement of DNA in bacteria. In fact, some theories suggest mitochondria were microorganisms that formed a symbiotic relationship with cells way back in our evolutionary past.
mtDNA often remains, even after nuclear DNA has completely broken down and can no longer be profiled. This is particularly useful for obtaining profiles from samples such as bones, teeth or hair, often recovered from the remains of long dead individuals, unidentified at the point at which they are discovered.
mtDNA is maternally inherited. Consequently all maternally related individuals will usually share the same mtDNA profile. This can be useful in identification because a reference sample from the suspected person’s mother (or other maternal line relative) can be used for comparison. However, whilst there may be many different mtDNA profiles in the general population, there are far fewer than the vast number of different nuclear DNA profiles. This often means that a mtDNA match may have less discriminating power, hence evidential significance than a nuclear DNA match. (See my post on match probabilities for a better idea of the meaning of discriminating power.)
The mtDNA technique was used to identify Russian remains, thought to be those of the Romanov family. mtDNA has also been used in an attempt to compare DNA from a bone fragment found on a South Pacific island, thought to be from missing aviator, Amelia Earheart, to DNA recovered from some of her correspondence, although results are inconclusive at this stage.
Y-STR is a technique that examines regions of the Y chromosome. This is useful forensically, particularly in sexual offence cases, because the Y chromosome only exists in males.
Imagine a scenario where intimate swabs from the complainant in a rape case have been examined. Finding semen on these swabs and obtaining a match to the suspect would be very good evidence to support the rape allegation. However, imagine that the seminal fluid detected on such swabs lacks spermatozoa (sperm cells.). This is not an unusual finding in rape case investigation if the offender has been vasectomised or has a low sperm count for medical reasons. The lack of sperm poses a potential problem because the vast majority of the DNA in semen is found in the sperm cells. There may be a very small amount of DNA from the offender in the seminal fluid, contained in other cells shed from the lining of his urethra, but these cells are visually indistinguishable from the vast numbers of female vaginal cells one would expect to find on the complainant’s intimate swabs in such a case. It becomes virtually impossible to separate the small number of male cells from the vast amount of female cells in such a sample. A colleague of mine compared the task to pouring a glass of water into the English Channel then trying to recover those particular water molecules.
The Y-STR technique allows profiling of the entire sample with no need to separate the male and female cells first. The Y-STR reaction targets the Y chromosome only, ignoring the female DNA that makes up the vast majority of the sample, and produces a profile solely from the Y chromosome.
The UK Forensic Science Service used Y-STR in a 1995 cold case – the first time the technique had been used to help achieve a conviction. Graham Darbyshire was sentenced to life imprisonment for a rape in Blackburn, Lancashire.
The limitations of the technique are similar to those for mtDNA. The Y chromosome is inherited paternally, therefore there may be fewer unique Y-STR profiles than nuclear DNA profiles in the general population, hence reducing discriminating power of the technique. Y-STR can be effective when considering a closed population. For example, in a case I worked on, an alleged sexual assault took place on a boat. Seminal fluid but no sperm cells were found on the complainant’s swabs and a Y-STR profile was attempted in order to link the seminal fluid to one of the limited number of males on the boat. In that instance, discriminating power was overridden by case circumstances.
The Canadian private sector forensic provider, Wyndham Forensic Group are currently at the validation stage of their Y-STR technique. They tell me via Twitter, that they’re planning an additional and novel use for the technique in helping to determining numbers of contributors to DNA mixtures. I’ll be interested to see how that works out. Check out @WyndhamForensic on Twitter for an update.
Familial searching is not really a new DNA technique at all. DNA profiling is carried out using the same methods as in other routine DNA cases. The difference comes when a DNA database search yields no matches. In a familial search, similar but non-matching profiles from the database are considered. Police investigators are supplied with this list of potential relatives to use as further intelligence to investigate the identity of a perpetrator.
The UK Forensic Science Service pioneered the technique in 2002 whilst re-investigating the 1973 rape and murder of Pauline Floyd, Geraldine Hughes and Sandra Newton. A profile was obtained during a cold case examination of retained materials from the original case. Although no DNA database match was obtained, analysts noticed a near match. Police, using a list of 100 names from this familial search, later identified Joe Kappen as the perpetrator. Although Mr Kappen had died by 2002, exhumation and sampling of his remains confirmed that his profile was a match to that obtained from the cold case materials.
Although privacy concerns have been raised by civil rights campaigners, familial searching is now being implemented across the US. In California, it was instrumental in apprehending the notorious serial killer known as the Grim Sleeper. The technique is also in use in Colorado and Virginia, and is considered in Pennsylvania and Minnesota.
So called ‘manufactured’ DNA was discussed at length in Cold Case Live Chat on March 11, 2011, and in the comments to my last guest post, so I thought it warranted further consideration here.
As forensic scientists, we’re taught that context is everything. It determines the evidential significance of the scientific findings in a case and often leads to the right questions being answered. It’s absolutely vital in a DNA case to consider not only who the DNA matches, but how the DNA was deposited. Failure to consider this issue has the potential to lead to a miscarriage of justice.
In late 2009, Israeli scientists reported successfully manufacturing a DNA profile in the lab. They took a blood sample and removed all of its DNA-containing cells. Next, they amplified a DNA profile from a man’s hair and mixed this with the DNA-free blood. The modified blood sample was sent to an independent US laboratory for testing, who detected the male profile and attributed it to the blood sample. The Israeli researchers also explained that a library of DNA snippets stored at the lab could also be used to fabricate any DNA profile in this way.
Whilst, in my view, the average criminal is unlikely to have access to sufficient laboratory equipment to manufacture DNA profiles at will, this development certainly raises questions about the validity of some DNA evidence, and corruption and the abuse of power cannot be discounted. Fortunately, the Israeli researchers also point out that such fabricated DNA lacks particular molecules (methyl groups) at certain points along the length of the DNA, and a test that can distinguish real from fabricated DNA is in development.
Phenotypic Markers and Ancestry DNA
So far, the DNA techniques discussed here examine regions of DNA scientists refer to as ‘junk DNA.’ In other words, these regions don’t contain any genes, so hold no information about an individual’s characteristics or appearance. (Such characteristics are referred to by scientists as phenotypes.) But what’s to stop scientists delving further into the information contained in DNA? The information is there, we just don’t look for it in a forensic context, yet…
Scientists at Columbian College, Washington DC, are investigating just that. They envisage the forensic use of a selection of ‘phenotypic markers’ containing information about hair colour, eye colour and the height of a person, amongst others. Scientists at the Penn Center for Bioethics are carrying out similar work with their Forensic DNA Phenotyping project.
Along the same lines, Sorensen Forensics of Salt Lake City recently launched ‘Investigative Law Enforcement Ancestry DNA’ (Investigative LEAD) testing. This technique examines specific ancestry markers within a DNA sequence and compares them to five major reference populations: Western Europe, Eastern Asia, Western Africa, the Indian Subcontinent and Indigenous America. The technique should help to identify the genetic ancestry of an unknown person and may be useful in cold cases, or in the identification of human remains that are particularly deteriorated.
Such considerations of ancestry are not a new idea. In 1999, a Dutch scientist illegally attempted to determine ancestry in the case of the murderer of a 16 year old girl. Whilst his attempts did not successfully identify the perpetrator, his conclusion that the offender was probably from North West Europe, allayed fears that the murderer was amongst a group of Kurdish, Iraqi and Afghan asylum seekers, living at a nearby hostel.
The UK Forensic Science Service has offered an ethnic inferencing technique for a number of years. FSS scientists first published a study of the feasibility of inferring ethnic origin from DNA profiles in the Journal of The Forensic Science Society as early as 1992. A paper reporting their effective use of the technique in a forensic context was finally published in August 2001 [Lowe et al, Forensic Science International 119 (2001) 17-21] In this paper, they are careful to point out its limitations. Accuracy of the technique relies on the integrity of the information contained within the racial databases used for comparison. However, said integrity assumes that sample information submitted by multiple investigators is accurate in terms of the ethnicity of each sample. The technique should therefore allow that this assumption cannot be reliably made. They also point out that, like any DNA comparison, the results are probabilistic and never definitive.
Similar ethical discussions seem now to be breaking out across the US and elsewhere. The Council for Responsible Genetics (CRG) in Cambridge, Massachusetts and GeneWatch UK recently collaborated to produce a worldwide register of DNA databases and comprehensive legal information, in order to monitor standards and advocate quality and consistency. It seems very clear that this complex discussion has only just begun. Let’s hope that world governments give it careful consideration and make some sensible decisions.